Solvent composition for liquid scintillation counting

ABSTRACT

A SOLVENT COMPOSITION FOR DISSOLVING SAMPLES FOR LIQUID SCINTILLATION COUNTING CONSISTING ESSENTIALLY OF (A) A SOLVENT SUITABLE FOR USE IN LIQUID SCINTILLATION COUNTING, (B) AT LEAST ONE QUATERNARY AMMONIUM BASE SELECTED FROM THE GROUP OF THE FORMULA:   (R&#39;&#39;-N(-X)(-R&#34;)-CH3)(+) (-)O-Y   WHEREIN R&#39;&#39; AND R&#34; ARE ALKYL RADICALS OF FROM 4 TO 18 CARBON ATOMS, X IS SELECTED FROM THE GROUP CONSISTING OF METHYL AND BENZYL RADICALS, AND Y IS SELECTED FROM THE GROUP CONSISTING OF HYDROGEN AND LOWER ALKYL RADICALS, THE PREDOMINANT CLASS OF QUATERNARY AMMONIUM CONSISTING OF THE GROUP WHEREIN R&#39;&#39; AND R&#34; CONTAIN FROM 4 TO 14 CARBON ATOMS, (C) METHANOL, AND (D) AN ALCOHOL SELECTED FROM THE GROUP CONSISTING OF T-PENTYL ALCOHOL AND 2-METHOXY ETHANOL. THE QUATERNARY AMMONIUM BASE PREFERABLY IS DIDODECYLDIMETHYLAMMONIUM HYDROXIDE OR METHOXIDE, AND IS PRESENT IN AT LEAST 0.25 MOLAR CONCENTRATION. THE SOLVENT ADVANTAGEOUSLY IS TOLUENE, BENZENE, XYLENE, OR DIOXANE, AND IS PRESENT IN 60-70% BY VOLUME. THE T-PENTYL ALCOHOL OR 2-METHOXY ETHANOL PREFERABLY ARE PRESENT IN 410% BY VOLUME.

US. Cl. 252-364 4 Claims ABSTRACT OF THE DISCLOSURE A solvent composition for dissolving samples for liquid scintillation counting consisting essentially of (a) a solvent suitable for use in liquid scintillation counting, (b) at least one quaternary ammonium base selected from the group of the formula:

wherein R and R" are alkyl radicals of from 4 to 18 carbon atoms, X is selected from the group consisting of methyl and benzyl radicals, and Y is selected from the group consisting of hydrogen and lower alkyl radicals, the predominant class of quaternary ammonium consisting of the group wherein R and R" contain from 4 to 14 carbon atoms, methanol, and (d) an alcohol selected from the group consisting of t-pentyl alcohol and 2-methoxy ethanol. The quaternary ammonium base preferably is didodecyldimethylammonium hydroxide or methoxide, and is present in at least 0.25 molar concentration. The solvent advantageously is toluene, benzene, xylene, or dioxane, and is present in 60-70% by volume. The t-pentyl alcohol or Z-methoxy ethanol preferably are present in 4- by volume.

BACKGROUND OF THE INVENTION Field of the invention My invention relates to a novel solvent composition for dissolving samples for scintillation counting, for example blood, urine, tissue homogenates and the like.

The prior art The counting or measurement of radioactivity by the liquid scintillation technique is now in widespread use in biological research and similar endeavor, where radioactive isotopes, such as beta-emitting isotopes such as tritium, carbon-l4, phosphorus-332 and sulfur-35 are employed as tracers. The general technique, as is well known, involves direct incorporation of the sample in a liquid scintillation composition, the scintillations being counted constituting a. highly efficient indication of the activity of the sample.

In general, the liquid scintillation technique basically involves the use of a suitable fluorescent substance, i.e. a fluor, disposed in a suitable solvent. The solvent, in addition to being required to dissolve the fluor, has further properties which are required in connection with the detection process. The energy of the particle being detected is transferred to the solvent by ionization or other excitation, and it is the induced excitation energy of the solvent which produces the excitation of the fluor. Additionally, the properties of the solvent must be such as to maximize the efiiciency with which the light produced by the fluor is transmitted to the light detector or detectors used to convert the scintillation to an electrical pulse. The solvent thus is desirably optically clear and has no substantial color 3,677,954 Patented July 18, 1972 characteristics which produce attenuation of the color characterizing the emission of the fluor. In addition to these fairly obvious optical effects which must be avoided, the excitations of the solvent are subject to being quenched in varying degrees by other processes which may occur.

There are now well known a substantial group of liquids meeting the general requirements discussed above, which are generally employed as primary solvents in liquid scintillation solutions. The most efficient of these in common use are the alkylbenzenes, toluene and xylenes (the purified ortho, meta or para forms or mixtures thereof). Benzene and mono-, diand triethylbenzenes and phenylcyclohexane are comparable in efliciency. Certain alkoxyethers, including anisole and dimethoxybenzene, have only slightly less efliciency. A further primary solvent, p-dioxane, having an efiiciency about 70 percent that of toluene, is frequently employed where miscibility with water is required. Other primary solvents which have been used in liquid scintillation counting are alkylbenzenes such as cumene, p-cymene, n-butylbenzene, mesitylene and 1,2,4- trimethylbenzene; ethers such as diphenylether, dimethoxyethane and ethoxybenzene; various other alkylated aromatic compounds such as monoisopropylbiphenyl and l-methylnaphthalene; hexane and DecalirijIn such solutions, of course, the fluor is the primary solute, many of which are likewise well known. The choice of primary solvent and primary solute depends upon a variety of factors, both as regards the equipment employed for detection of the light scintillations (wave length sensitivity, etc.) and as regards the specific characteristics of the radiation under measurement and similar factors, particularly the nature of the material to be dissolved or distributed in this primary solution for the measurement of its radioactivity.

Where a sample to be counted is readily soluble in various primary solvents, preparation for counting is obviously simplest.

Where the sample cannot be directly dissolved in a primary solvent in adequate concentration, it becomes necessary to employ a secondary solvent, in which the sample may be dissolved, and the resulting solution is thereupon dissolved in the primary solution. Various secondary solvents are used for this purpose, such as alcohols, ethers, etc. In general, however, the use of a secondary solvent somewhat increases the quenching, i.e., reduces the efiiciency of the overall process.

One type of sample, which often cannot be directly dissolved in a primary solvent and thus is desirably solubilized with a particular type of secondary solvent, which is very commonly encountered, is animal tissue. These may be digested by use of a strong base. Likewise solubilization difliculties in primary solvents have arisen with aqueous samples containing phosphate, sulfate or carbonate ion. Various strong-base solutions have been used as secondary solvents for such samples. One type of strong base solution for dissolving these types of samples is that of a quaternary ammonium hydroxide in methanol. Previously, the most commonly used hydroxide for this purpose was p-(diisobutyl) cresoxyethoxyethyldimethylbenzyl ammonium hydroxide, generically termed methylbenzenethonium hydroxide, and commonly referred to in the literature as the hydroxide of Hyamine lO-X (trademark of Rohm & Haas). Such a preparation is made from the corresponding halide by a published procedure called the silver oxide process, see, e.g., Liquid Scintillation Counting, edited by Bell and Hayes, Pergamon Press, 1958, pages 123 et seq.

In US. Pat. 3,506,828 of Hansen and Bush there is disclosed a solvent composition wherein a particular group of quaternary ammonium compounds is proposed as an improvement over the compositions described above, such compounds having the formula:

wherein R and R" are alkyl radicals of from 4 to 18 carbon atoms, X is selected from the group consisting of methyl and benzyl radicals, and Y is selected from the group consisting of hydrogen and lower alkyl radicals, the predominant class of quaternary ammonium consisting of the group wherein R and R" contain from 4 to 14 carbon atoms.

SUMMARY OF THE INVENTION In accordance with my invention there is provided a novel solvent composition for dissolving samples for liquid scintillation counting, which exhibits substantially enhanced dissolving power compared to the compositions of U.S. 3,506,828.

My novel composition consists essentially of:

(a) a solvent suitable for use in liquid scintillation counting, advantageously toluene, benzene, xylene, p-dioxane, or mixtures thereof, desirably 60-70% by volume,

(b) at least one quaternary ammonium base selected from the group of the above formula I, desirably in at least 0.25 molar concentration, and up to 1.0 molar or higher,

(c) methanol, desirably 15-25% by volume, and

(d) an alcohol selected from the group consisting of t-pentyl alcohol and 2-methoxy ethanol, desirably 4-10% by volume.

The quaternary ammonium base (b) desirably is didodecyldimethyl ammonium hydroxide, but other bases can be used such as ditallowdimethyl-, dilauryldimethyl-, dioctylbenzylmethy1-, didecylbenzylmethyl-, dicocodimethyl-, didecyldimethyl-, and dioctyldimethyl ammonium hydroxides. The compositions of the ditallowand dicococompounds are described in detail in U.S. 3,506,828.

Various processes for preparing such compounds are described in U.S. 3,506,828, for example anion exchange, silver oxide method, alcoholic potassium hydroxide reaction, barium hydroxide method, and the alkoxide reaction.

While the quaternary compound of the composition is nominally the hydroxide, the presence of methanol may cause the compound to exist as an equilibrium mixture of the hydroxide and methoxide.

THE SPECIFIC EMBODIMENTS Example I A solubilizer is prepared by reacting 30 grams of didodecyldimethyl ammonium bromide, 80 ml. of a solution containing 80% toluene and 20% methanol by volume, and 4 grams of solid potassium hydroxide at room temperature. (The solution is previously decolorized and scavenged of oxygen by adding traces of zinc and stannous chloride which are then filtered out.) After standing overnight the clear supernatent liquid is decanted, to provide 90 ml. of product which is then diluted to 100 ml. with 3 ml. of methanol and 7 ml. of t-pentyl alcohol. The resultant novel solubilizer comprises about 64% toluene, 19% methanol, and 7% t-pentyl alcohol by volume, and has a 0.6 molar concentration of didodecyldirnethylammonium hydroxide.

The product is evaluated by placing 2 ml. in a vial and treating it with 0.1 ml. of water and 10 ml. of a standard liquid scintillation counting solution containing 4 grams of PPO (2,5-diphenyl oxazole) and 0.5 gram of POPOP (1,4-di-2-(S-phenyloxazolyl)benzene) dissolved in 1 liter of toluene.

The vials are shaken to obtain clear homogeneous solutions and then cooled to C. Successive 0.1 ml. increments of water are added, shaken, and observed until phase separation occurs. By this test it is found that with my solubilizer composition phase separation does not occur until 0.6 ml. of water has been added.

In contrast, when using a commercial similar solubilizer, without t-pentyl alcohol (as described in U.S. 3,506,828), a haze appears at 0.4 ml. of water, and definite phase separation occurs at 0.5 ml. of water. Moreover, more time and more vigorous shaking are required to obtain solution of each increment of water than with my novel solubilizer.

Furthermore, my quaternary ammonium compound solution as described above, but without t-pentyl alcohol, successfully solubilizes water only up to 0.3 ml. Where phase separation occurs.

Thus it is evident that my novel solubilizer provides an improvement in solubilizing efiect of 20% over the prior art comparison sample; and over my product from which t-pentyl alcohol has been omitted.

Moreover, my solubilizer gives a lower background count than does the comparison commercial solubilizer.

Example II The commercial similar solubilizer described above is modified by adding thereto 7% of t-pentyl alcohol by volume. Its solubilizing ability is tested in the same manher and is enhanced to about the same extent as in Example I.

Example III In a test similar to Example I, but using 14% methanol, 69% toluene, and 7% t-pentyl alcohol, effective solubilization of water is obtained.

Example IV Following the procedure of Example I, final dilutions are accomplished with t-pentyl alcohol, and with 2-methoxyethanol, to produce final solubilizer products containing 4% and 6% by volume (with 22% and 20% methanol being present, respectively, along with 64% toluene). With all four compositions water is successively solubilized up to 0.6 ml.

The invention has been described in detail with particular reference to the preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim:

1. A solvent composition for dissolving samples for liquid scintillation counting consisting essentially of, in percent by volume,

(a) about 60-70% of a solvent suitable for use in liquid scintillation counting, other than methyl alcohol, t-pentyl alcohol and 2-methoxy ethanol,

(b) at least 0.25 molar concentration of at least one quaternary ammonium base selected from the group of the formula:

wherein R and R" are alkyl radicals of from 4 to 18 carbon atoms, X is selected from the group consisting of methyl and benzyl radicals, and Y is selected from the group consisting of hydrogen and lower alkyl radicals, the predominant class of quaternary ammonium consisting of the group wherein R and R" contain from 4 to 14 carbon atoms,

(c) about 15-25% of methanol, and

(d) about 4-10% of an alcohol selected from the group consisting of t-pentyl alcohol and 2-methoxy ethanol.

2. A solvent composition in accordance with claim 1 6 wherein said solvent is at least one selected from the group References Cited consisting of toluene, benzene, xylene, and dioxane UNITED STATES PATENTS 3. A solvent composition in accordance with claim 1 wherein said quaternary ammonium base is predominant- 3,506,828 4/1970 Hansen et a1 252 364 X 1y didodecyldimethylamrnoniurn hydroxide or methoxide. 5 2362444 12/1958 Drautz 252 3 64 X 4. A solvent composition in accordance with claim 3 containing, in percent by volume, about 64% of toluene, RICHARD LOVERING Primary Exammer about 19% of methanol, and about 7% of t-pentyl alcohol; U S C1 X R and containing didodecyldimethylammonium hydroxide or methoxide in about 0.6 molar concentration. 10 252-4635, 408 

